1 //===-- Implementation of crt for x86_64 ----------------------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 
9 #include "config/linux/syscall.h"
10 #include "include/sys/syscall.h"
11 
12 #include <linux/auxvec.h>
13 #include <stdint.h>
14 
15 extern "C" int main(int, char **, char **);
16 
17 struct Args {
18   // At the language level, argc is an int. But we use uint64_t as the x86_64
19   // ABI specifies it as an 8 byte value.
20   uint64_t argc;
21 
22   // At the language level, argv is a char** value. However, we use uint64_t as
23   // the x86_64 ABI specifies the argv vector be an |argc| long array of 8-byte
24   // values. Even though a flexible length array would be more suitable here, we
25   // set the array length to 1 to avoid a compiler warning about it being a C99
26   // extension. Length of 1 is not really wrong as |argc| is guaranteed to be
27   // atleast 1, and there is an 8-byte null entry at the end of the argv array.
28   uint64_t argv[1];
29 };
30 
31 // TODO: Would be nice to use the aux entry structure from elf.h when available.
32 struct AuxEntry {
33   uint64_t type;
34   uint64_t value;
35 };
36 
37 extern "C" void _start() {
38   uintptr_t *frame_ptr =
39       reinterpret_cast<uintptr_t *>(__builtin_frame_address(0));
40 
41   // This TU is compiled with -fno-omit-frame-pointer. Hence, the previous value
42   // of the base pointer is pushed on to the stack. So, we step over it (the
43   // "+ 1" below) to get to the args.
44   Args *args = reinterpret_cast<Args *>(frame_ptr + 1);
45 
46   // After the argv array, is a 8-byte long NULL value before the array of env
47   // values. The end of the env values is marked by another 8-byte long NULL
48   // value. We step over it (the "+ 1" below) to get to the env values.
49   uint64_t *env_ptr = args->argv + args->argc + 1;
50   uint64_t *env_end_marker = env_ptr;
51   while (*env_end_marker)
52     ++env_end_marker;
53 
54   // After the env array, is the aux-vector. The end of the aux-vector is
55   // denoted by an AT_NULL entry.
56   for (AuxEntry *aux_entry = reinterpret_cast<AuxEntry *>(env_end_marker + 1);
57        aux_entry->type != AT_NULL; ++aux_entry) {
58     // TODO: Read the aux vector and store necessary information in a libc wide
59     // data structure.
60   }
61 
62   __llvm_libc::syscall(SYS_exit,
63                        main(args->argc, reinterpret_cast<char **>(args->argv),
64                             reinterpret_cast<char **>(env_ptr)));
65 }
66